1. Field of the Invention
The present invention generally relates to a semiconductor device, and more particularly to a field effect transistor.
2. Description of the Related Art
Presently, high dielectric constant gate dielectrics for silicon complementary metal oxide semiconductor (CMOS) devices, such as transistors, typically utilize a silicon dioxide gate dielectric. Other gate dielectrics employed in manufacturable devices have contained a silicon oxynitride layer as part of the gate dielectric stack as well. As CMOS devices miniaturize, scaling laws require that the parameter e/d, where e and d are the permittivity and thickness of the dielectric layer respectively, reduce as well. For a fixed gate dielectric material such as silicon dioxide, where the permittivity is 3.8, its thickness therefore must reduce as devices become smaller. However, below a physical thickness of approximately 1.5-1.7 nanometers, the layer starts transmitting an unacceptably high amount of electrical leakage current through it.
An additional, secondary problem that arises when the dielectric layer becomes so thin, is that it also becomes impervious to the diffusion of impurities, or dopant atoms, through it. As a result, such a dielectric layer fails to protect the underlying silicon substrate below it.